1. Field of the Invention
The present invention relates to medical methods and systems, and more particularly to methods and systems for estimating body fluid metrics.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The maintenance of body fluid balance is of foremost concern in the care and treatment of critically ill patients, yet physicians have access to few diagnostic tools to assist them in this vital task. Patients with congestive heart failure, for example, frequently suffer from chronic systemic edema which must be controlled within tight limits to ensure adequate tissue perfusion and prevent dangerous electrolyte disturbances. Conversely, dehydration from diarrhea is one of the most common causes of hospitalization of infants and children and can be life-threatening if not recognized and treated promptly. Additionally, hospital admission of long-term care patients due to dehydration occurs with alarming frequency, high costs to the health care system, and frequently with poor outcomes for patients.
The most common method for judging the severity of edema or dehydration is based on the interpretation of subjective clinical signs (e.g., swelling of limbs, dry mucous membranes), with additional information provided by measurements of the frequency of urination, heart rate, serum urea nitrogen (SUN) to creatinine ratios, and blood electrolyte levels. However, none of these variables alone is a direct and quantitative measure of water retention or loss.
An indicator dilution technique, which provides the most accurate direct measure of water in body tissues, is the present de facto standard for assessment of body fluid distribution. However, indicators can take up to four hours to equilibrate in an adult, and require careful collection of fluid samples and extensive equipment for precise analysis, making the technique inconvenient for a clinical setting.
Alternatively, a great deal of research has been conducted regarding an electrical impedance technique, which involves the use of electrical impedance monitors for measurement of total body water. This electrical-impedance technique is based on measuring changes in the high-frequency (typically 10 KHz-1 MHz) electrical impedance of a portion of the body. However, while results of using the electrical-impedance technique for a single tissue have proven to be clinically useful, mixed results have been obtained with the electrical-impedance technique in clinical studies of whole body fluid disturbances, as reported by various investigators. The results generally indicate that the electrical-impedance technique can be inaccurate when measuring total body water. The rather poor accuracy of the technique seen in many studies points to unresolved deficiencies when applied in a clinical setting.